CFD and underwaterfilming not only confirmed that pretwisted fins are needed but it also showed how much and what other influences there are to the water flow. It depends on the bottom shape, position of the fin and the average speed of a board. And also asymmetrical fins need to have the angles adapted. Nowadays CFD programs have become very accurate and offer various advantages over drag tank testing: It is possible to “see” the water flow directions and speeds everywhere and measure the performance of each fin separately rather than just being able to feel the fin cluster as a whole.
Apart from drag another important issue to have flow optimised side fins is turning. If the side fins are not set up with enough angle, they will be giving outward lift even without putting pressure on them. The ideal would be to set the side fins up so that when there is no sideways force applied from the sailor or sail, they simply cut through the water without providing any lift. This is how it should be, it gives the least drag and is the ideal starting situation to turn. In regular B&J sailing, when comparing to a single fin set up, the trifin proved even to be faster on any course, probably due to the increased control. As fins are small, this may seem insignificant but remember fins handle the same amount of power as a sail which is much bigger.
Then when a force is applied by the sailor or sail, all fins will do a similar amount of work:
The small differences stem from the position and asymmetry of each fin.
Paralel placed symmetrical side fins do give lift even when no pressure is applied by the sailor:
Then, when in a turn the outer fin ventilates and thus losing lift, the inner fin will be steering against the turn, making the board want to straight line.
Computer generated images of a bottom turn to show how realistic CFD technology can simulate situations like this nowadays. CFD also gives images of flow directions, flow speeds, pressure distribution over the board and fins, ventilation, tip vortexes, water and air mixtures and points of increased turbulence or drag but we are not going to give it all away…..
Since fin lift increases exponentially with the speed, you will notice this a lot more the faster you go and less at lower speeds.
So the ideal starting situation for entering a turn is that none of the fins are giving lift when there is no pressure applied to them, like a centre fin already does naturally. From this as soon as any pressure is applied to the fins, each will do an equal amount of “work”.
All profiles are developed to handle the widest range of Angle of attack as possible. Essential for wave sailing plus you are able to choose smaller fins and they are easier to get planing.
The side fins have a wing like cambered asymmetrical profile so the inner fin can handle higher pressures at lower drags in a turn. Very important for using asymmetrical foils is to adapt the angle of attack since an asymmetrical foil will provide lift already with zero angle of attack, like airplane wings do as well. So you need to readjust the angle to where the fin has the least drag and does not provide lift again. The amount of adjustment depends on the amount of asymmetry. Without the use of CFD this is very hard to find by trial and error since you can only feel what the whole fin cluster is doing, not each fin separately.
The above picture also illustrates why a tri fin set up provides more predictability/control. If one of the side fins is ventilated, you only lose 25 to 33% of fin area holding the board in the turn. Would you want the tail to break out sooner, you could put a smaller centre fin and bigger side fins.
All fins are swept back so they adapt better to the situation, handle tight turns better and are generally easier to sail. Compare the amount of fin flex of a carbon fin in a soft top turn: